Red shells are a variety of shell found in the Precipice in Rayman 2 and its remakes, as well as in Vertigo Wastes in the Game Boy Advance and N-GAGE versions of Rayman 3. Unlike regular walking shells, these ones cannot be tamed and ridden, and are always hostile towards Rayman.
In the Precipice, red shells are of the walking kind. They are fired in special containers into Rayman's path by the Robo-Pirates' warship. When these containers land, they break open, and the red walking shell charges out of them towards Rayman. They cannot turn, but they can run over empty spaces. They can be destroyed by Rayman's magic fist, or by crashing into any object, such as Rayman or another red shell. In most cases, the easiest way for Rayman to avoid them is to jump over them and let them run past.
In Vertigo Wastes, the red shells have flying abilities. They stand on ledges and wait until Rayman comes near, then lift off and launch toward him. Here too, Rayman can simply jump to let them fly by.